devcontainer.json: add support for multi-platform "localEnv" in "mounts"

First and foremost: thank you guys for your amazing work with VSCode!

Use Case

I have a devcontainer.json that on Windows mounts "/home/nwtimetracking/" to "C:\Users\Desktop<my_username>\nwtimetracking".

The problem arises when I load the project on Linux, where I do get a "A mount config is invalid" error message and I can't proceed until I manually switch USERPROFILE with HOME.

devcontainer.json
https://github.com/numbworks/nwtimetracking/blob/master/.devcontainer/devcontainer.json

{
	"build": { 
		"dockerfile": "Dockerfile" 
	},
	"customizations": {
		"vscode": {
			"settings": {},
			"extensions": [
				"ms-python.python",
				"ms-python.vscode-pylance",
				"ms-toolsai.jupyter",
				"ms-toolsai.datawrangler",
				"shd101wyy.markdown-preview-enhanced"
			]
		}
	},
    "mounts": [
        "source=${localEnv:USERPROFILE}/Desktop/nwtimetracking/,target=/home/nwtimetracking/,type=bind,consistency=cached"
    ]	
}

As far as I know and at the moment of writing:

• {localEnv:USERPROFILE} works only for Windows
• {localEnv:HOME} works only for Linux/MacOS

Feature Request

If would be nice to have some kind of conditional logic that allows the use of both dynamically, according to the OS on which VSCode is running at the moment. Or a combo label that identifies the DESKTOP on all the supported OSes. Or similar solution.

Or if you are aware of a workaround for this issue, I would be glad to know!

Thank you!